Ive got a 1991 f150 4x4 with the 4.9 and she's giving me and intermittent check engine light. I pulled the codes KOEO with my inovva scanner and got a 111 pass 10 separation then a 181 continuous memory code. One issue I am experiencing is the truck will occasionally buck in 4th gear while cruising 1750-2000k on the tac. I know the code means "(Trucks only) HEGO (HO2S) sensor fault unable to switch / lean during part throttle" I have checked key on fuel pressure and running fuel pressure and all is within spec of the 4.9 Ive read that the 181 code is often caused my an exhaust leak before the o2 sensor which by the way I replaced a month ago with a new Bosch unit. I have an exhaust leak but it is at the in port on the muffler don't think that would cause this issue. Another thing is the truck always smells like it is running rich from the exhaust fumes. The fuel pressure reg is not leaking i have checked that the filter is new both tanks work and cause the same issue. Could I be looking at a cracked manifold bad donut gasket or? if so any tricks to this as it looks pretty difficult all the studs are very rusty down there and I'm sure they will snap off! just don't know thought maybe someone could point me in the right direction so i don't just waste money on parts i don't need. Thanks!

 

What were the fuel PSI values at idle/idle with the vacuum line removed/under load?